(1) Field of the Invention
The present invention relates to the manufacture of semiconductor memories, and in particular, directed to a multi-level stacked gate flash memory cell of 4 state/2-bit storage capability and to a method of forming the same.
(2) Description of the Related Art
The present state of the art for increasing memory density in semiconductors is to reduce the size of the memory cell. Another approach to improving memory density is to increase the number of possible states in a cell. The multi-level concept is applicable to both volatile and nonvolatile memories, but has been difficult to implement at a commercial level. It is disclosed later in the embodiments of this invention a method of forming a highly dense stacked gate flash memory cell with a structure having multi floating gates that can assume 4 states and, therefore, store 2 bits at the same time.
The disclosed multi-bit storage takes advantage of the analog nature of the flash storage element. The conventional 1 bit/cell approach would place the cell in one of two states, a “0” or “1”, using a program or erase operation. Erase might be denoted as the absence of charge and program as the presence of charge on the floating gate. Thus, the cell is placed in one of two discrete charge bands. If programming can be done accurately enough, the cell can be placed in one of four discrete charge bands, or, states, achieving 2 bits/cell storage. A novel state assignment for the disclosed cell will be shown later in the embodiments of this invention.
Memory devices include the earlier electrically erasable and electrically programmable read-only memories (EEPROMs), and now, the flash electrically erasable and electrically programmable read-only memories (flash EEPROMs). Generally, flash EEPROM cells having both functions of electrical programming and erasing may be classified into two categories, namely, a stacked-gate structure and a floating gate structure, which is not discussed here. A conventional stacked-gate type cell is shown in FIG. 1a where, as is well known, tunnel oxide film (20), a floating gate (30), an interpoly insulating film (40) and a control gate (50) are sequentially stacked on a silicon substrate (10) between a drain region (13) and a source region (15) separated by channel region (17). Substrate (10) and channel region (17) are of a first conductivity type, and the first (13) and second (15) doped regions are of a second conductivity type that is opposite the first conductivity type. FIG. 1b shows an electrical schematic of the stacked gate cell of FIG. 1a with correspondingly primed numerals.
The programming and erasing of the flash EEPROM shown in FIG. 1a is accomplished electrically and in-circuit by using Fowler-Nordheim (F-N) tunneling. Basically, a sufficiently high voltage is applied to control gate (50) and drain (13) while source (15) is grounded to create a flow of electrons in channel region (17) in substrate (10). Some of these electrons gain enough energy to transfer from the substrate to control gate (50) through thin gate oxide layer (20) by means of (F-N) tunneling. The tunneling is achieved by raising the voltage level on control gate (50) to a sufficiently high value of about 12 volts. As the electronic charge builds up on floating gate (30), the electric field is reduced, which reduces the electron flow. When, finally, the high voltage is removed, floating gate (30) remains charged to a value larger than the threshold voltage of a logic high that would turn it on. Thus, even when a logic high is applied to the control gate, the EEPROM remains off. Since tunneling process is reversible, floating gate (30) can be erased by grounding control gate (50) and raising the drain voltage, thereby causing the stored charge on the floating gate to flow back to the substrate.
The programming of the stacked-gate cell shown in FIG. 1a is performed on a single floating gate (30). It will be appreciated by those skilled in the art that the multi-floating gate disclosed in the present invention will provide multi-bit storage based on twice the two-states possible on each floating gate in a stacked-gate cell. In prior art, workers in the field have disclosed multi-level memory cells, but for split-gate memory cells only.
Thus, Liang, et al., disclose in U.S. Pat. No. 5,714,412 a multi-level flash memory cell as applied to a split-gate, but not to a stacked-gate flash memory cell. In their disclosure, a semiconductor memory device is formed on a doped semiconductor substrate, and covered with a tunnel oxide layer covered in turn with a doped first polysilicon layer. The first polysilicon layer is patterned into a pair of floating gate electrodes. An interelectrode dielectric layer covers the floating gate electrodes, the sidewalls of the floating gate electrodes and the edges of the tunnel oxide below the floating gate electrodes. A second polysilicon layer overlies the interelectrode dielectric layer and is in turn covered by a tungsten silicide layer. A second dielectric layer covers the tungsten silicide layer. A control gate electrode which spans the pair of floating gate electrodes is formed by the second polysilicon layer, the tungsten silicide and the first and second dielectric layers patterned into a gate electrode stack providing a control gate electrode spanning across the pair of floating gate electrodes. There are source/drain regions in the substrate self-aligned with the control gate electrode. U.S. Pat. No. 5,877,523 by the same inventors discloses the structure of the same multi-level, split-gate, flash memory cell.
A different PMOS flash memory cell capable of multi-level threshold voltage storage is disclosed in U.S. Pat. No. 5,666,307 by Chang. A P-channel flash EEPROM cell has P+ source and P+ drain regions, and a channel extending therebetween, formed in an N-type well. A thin layer of tunnel oxide is provided over the channel A poly-silicon floating gate and poly-silicon control gate, separated by a dielectric layer, overlie the tunnel oxide. Programming is accomplished via hot electron injection while erasing is realized by electron tunneling. The threshold voltage of the cell is controlled by the magnitude of voltage coupled to the floating gate during programming. PMOS devices conduct a gate current via hot electron injection over a narrow range of gate voltages, thereby allowing for precise control over the gate current and thus over the charging of the floating gate. This control over the gate current allows the threshold voltage of the cell to be more accurately controlled, thereby resulting in a more reliable cell capable of storing a greater number of bits of data.
Another invention by Lin, et al., in U.S. Pat. No. 5,851,881 provides a structure and a method of manufacturing split-gate MONOS multi-level logic memory device. The memory device has a poly stacked gate transistor in series with a MONOS (Metal-Oxide Nitride-Oxide Semiconductor) transistor. A tunnel oxide is first formed on the surface of a semiconductor substrate. The substrate has a stacked gate channel area and a MONOS channel area in the active regions. A poly floating gate electrode is formed over the stacked gate channel region. An ONO layer having a memory nitride layer is formed over the floating gate and the tunnel oxide layer over the MONOS channel region. A control gate electrode is formed over the ONO layer spanning across the poly floating gate electrode and the MONOS channel region. Source/drain regions are formed in the substrate. A poly flash transistor and a MONOS flash transistor combine to form the 4-level logic memory cell of the invention.
The multi-level logic memory cell disclosed in the instant invention is formed of multi-floating gates providing multi-bit storage based on twice the two-states possible on each floating gate in a stacked-gate cell.